According to the DVD standard (“DVD Specifications for Read-Only Disc, version 1.0”), it is provided that sub-picture data, mainly subtitles or DVD menu highlights, might be recorded on certain area(s) of the DVD. During playback of the DVD, said sub-picture data is decoded and displayed combined with decoded video data.
Sub-picture information is arranged in data units called Sub-Picture Units (SPUs). Sub-picture data is defined in DVD standard with interlaced display device in mind. Interlaced display is a type of video display in which one image is displayed in two successive sweeps by the electron beam. Odd lines (1, 3, 5, . . . ) are displayed first, followed by even lines (2, 4, 6, . . . ). Data corresponding to odd and even lines is known as top field and bottom field respectively. Moreover, the DVD standard specifies sub-picture data in such a way as to be easily decoded and displayed “on the fly”, that is to say without buffering into external frame buffer memories. This is the reason why top and bottom fields of a whole video frame are encoded separately inside a SPU (top field being recorded first) a frame designating data corresponding to a full image to be displayed on the screen. This feature of the SPU structure is referred to as “Field format” in the following.
Sub-picture decoder circuits which are readily available on the market are designed in accordance with the sub-picture data having the afore-mentioned field format. They are optimised to process (decode and display) field-based SPUs.
If, however, sub-picture data is to be displayed on a progressive display device, for instance a progressive TV, top and bottom fields have to be combined in one frame at some point before displaying it. It is recalled that progressive display is another type of video display in which all lines of one image (1, 2, 3, . . . ) are displayed sequentially over a single sweep, as opposed to interlaced display (see above).
The first solution coming in mind is to redesign a specific hardware for the sub-picture decoder circuit, so that the latter have a progressive output capability. Such solution is however cumbersome.